1. Field of the Invention
The invention relates to a high pressure swivel comprising an annular inner wall and an annular outer wall, rotatable relative to one another around an axis, the walls defining a toroidal chamber, an axial gap extending between the walls from the chamber to an outer part, the gap comprising seen in axial direction, toward the outer part:                an isolation seal situated in the gap adjacent the chamber,        an isolation gap section extending downstream from the isolation seal,        a primary seal situated in the gap,        a primary gap section situated downstream from the primary seal, and        a secondary seal situated downstream from the primary seal.        
2. Description of Related Art
High pressure swivels, used at pressures between 100 and 400 bar are known from U.S. Pat. No. 4,647,076.
Such swivels having multiple flow paths use in-line and toroidal couplings to convey fluids from a substantially fixed part to a rotating part. Commonly such swivels are used in the production of hydrocarbons on weathervaning, ship-shaped, floating production systems where flow lines transfer fluids between the floating unit and the seabed. To avoid the twisting of flow lines as the vessel weathervanes they are connected to a turret about which the vessel rotates. The swivel is located in the flow path between the fixed turret and the rotating vessel.
To allow for the swivel rotation, bearings and seals are used. The seals in these swivels are often made of synthetic elastomeric or plastic low friction materials that generally slide on smooth, closely machined, hard metallic surfaces. The properties of these seals and their ability to seal across the gap formed at the interface between the fixed and rotating swivel parts is well-known. The swivel designs take care that these swivel gaps, referred to as “extrusion gaps”, stay within the proven limits of the particular type of seal used in the design. These extrusion gaps limits for seal materials are known to vary with both temperature and pressure.